Seiso is a term borrowed from Japanese idol culture for performers presented as pure, polite and wholesome, and VTuber fans use it constantly as both a compliment and a joke. Bao, an English speaking VTuber known for playful and chaotic streams, plays against that image, which is why a short denial in Japanese became a shareable reaction clip. Fans cut lines like this into edits, Discord soundboards and short videos, where the phrase works even out of context as a knowing wink that the cute persona is not entirely innocent.